The invention relates to PCI and CompactPCI bus systems.
Modern communications systems typically utilize some type of electronic bus system to provide data communications between various computer components. A bus system is characterized in terms of a number of different properties, including electrical properties, timing properties, and data communication protocols.
Physically, a computer bus comprises a set of signal lines that extend in parallel between multiple peripheral connectors or xe2x80x9cslots.xe2x80x9d Peripheral devices, sometimes referred to as xe2x80x9cexpansion cards,xe2x80x9d can be plugged into the peripheral connectors. Examples of such peripheral devices include memory devices, mass storage interface devices, sound cards, graphics cards, other I/O cards, etc.
In many cases, the bus is part of a so-called xe2x80x9cmotherboard,xe2x80x9d which incorporates a microprocessor and other components for controlling operations on the bus. In other cases, the bus is implemented as a passive xe2x80x9cbackplane,xe2x80x9d and controlling components are located on a peripheral device plugged into one of the peripheral connectors. The individual signal lines of the bus system are driven by components on the motherboard and/or by similar components on different peripheral devices.
The PCI (Peripheral Connection Interface) bus is one popular example of a computer bus. The PCI bus is used in popular personal computers, in conjunction with a motherboard. A processor and other base components of a computer are located on the motherboard, and peripheral components are installed in PCI connectors on the motherboard to customize a computer to a particular user""s requirements. The PCI bus is a well known and widely accepted standard, defined in a document entitled xe2x80x9cPCI Local Bus Specificationxe2x80x9d. available from PCI Special Interest Group of Hillsboro, Oreg. The PCI bus will be referred to as the xe2x80x9cDesktopxe2x80x9d PCI bus in the following discussion.
The PCI bus satisfies the needs of a desktop computer, in which it finds its primary application. However, a variation of the PCI bus has also been adapted to more industrial environments. This variation, known under the trademarked term xe2x80x9cCompactPCIxe2x80x9d. borrows the electrical specification of the Desktop PCI bus. As an additional electrical feature, it adds the ability to xe2x80x9chot swapxe2x80x9d peripheral boardsxe2x80x94to change boards without removing power.
A further feature of the CompactPCI bus is that it allows processor boards, referred to as xe2x80x9csystemxe2x80x9d boards, to be plugged into the bus rather than residing on a motherboard. Thus, the CompactPCI bus is usually utilized in conjunction with a passive backplane, which accepts both peripheral boards and system boards. Physical specifications differ from those specified for the Desktop PCI bus; the sizes of peripheral boards and the connectors used are different. The CompactPCI bus uses a gas tight, high density pin-and-socket connector which meets the IEC-1076 international standard. This connector has a number of advantages, all of which are favorable to the use of the CompactPCI bus in industrial environments. CompactPCI boards have a form factor similar to that of the VME bus, which is another bus that is widely accepted in industrial control environments.
The CompactPCI bus is also well known and widely used, and is defined in a document entitled xe2x80x9cCompactPCI Specificationxe2x80x9d. available from PCI Industrial Computer Manufacturers Group (PICMG). PICMG is a consortium of industrial computer product vendors who collaboratively develop specifications for PCI-based systems and boards for use in industrial and telecommunications computing applications.
Prior to developing the invention, the inventors had proposed a development system based on the CompactPCI bus. This bus was desirable because it allowed different processor boards (and associated microprocessors) to be swapped in and out of the development system. Because of this, software developers would be able to use the development system to develop software for a system in which different microprocessors could be used.
A significant disadvantage of the CompactPCI bus, however, is that peripheral cards for this bus are less widely available and more expensive than Desktop PCI cards having the same functionality. Accordingly, the inventors realized that it would be desirable to be able to use Desktop PCI cards in the development system. Although bridging circuits were available for integrating CompactPCI and Desktop PCI buses, such circuits would have added expense, size, and complexity which the inventors wished to avoid. On the other hand, a non-bridged bus designed to accommodate a useful number of both CompactPCI and Desktop PCI peripheral cards would likely either violate PCI timing specifications or run at a reduced clock speed.
In accordance with the invention, a CompactPCI bus is implemented on a passive centerplane. CompactPCI connectors are positioned on one side of the centerplane. Desktop PCI connectors are placed on the other side. Both types of connectors are connected directly to a common electrical bus, which complies with the CompactPCI standard. This is possible because the Desktop and CompactPCI buses share common electrical characteristics. Rather than laying out the connectors side-by-side on a single side of the centerplane, the PCI and CompactPCI connectors are interleaved by placing them on opposite sides of the centerplane. Due to the mechanical restrictions of placing the connectors side-by-side, interleaving the connectors on opposite sides of the centerplane conserves space, allowing five Desktop PCI peripheral cards and three CompactPCI peripheral cards to be plugged into a single centerplane. The CompactPCI system slot board may be one to three units wide, while the CompactPCI peripheral slot boards may be one to two units wide. This design allows as many as eight PCI (CompactPCI or Desktop) peripheral cards to reside on the same bus, meeting PCI Electrical and physical requirements without requiring active bridging circuits.